USB-6501 Output Drive Configuration Issue

[Thomas_GERARDIN]

Hello,

I'm using a USB-6501 for my project, but I'm encountering an issue with the I/O configuration. By default, when the DAQ is connected to the PC, the output lines are set to active drive (push-pull), and thus remain at 5V due to the internal pull-up resistors (4.7k).

According to the datasheet, the output drive type is software-selectable. However, I haven’t found a way to change this setting, either through LabVIEW or directly via NI MAX.

How can I configure the DAQ outputs to be active low when the device is connected to the PC ?

Best regards !

Thomas

[L.Zakarian]

Hello Thomas,

There are indeed different ways to switch from active drive (push-pull) to open collector (open-drain) for your USB-6501.

Using NI MAX: When you are creating a new task for digital output, you can look for the option to set the output drive type. To do so, you should select the option “Open Collector” in the task configuration instead of “Active signal”.

Using LabVIEW: When you are creating a new DAQmx Task in LabVIEW, use the “DAQmx Create Channel” VI to specify the digital output lines. And then, after adding a “DAQmx Channel Property Node” VI you should set the output drive type property to “Open Collector”.

I hope it will solve your problem,

Louis Zakarian

[ZYOng]

Setting Default State of Digital Line with Pull-Down Resistors on a DAQ Device

USB-6008/6009/6501 have a default power up state of a high impedance input or alternatively called open collector/open-drain.

To set other values on the DIO lines, it must be statically updated by Software after power up of your system. If you have a need for programmable power-up states in your application, please look for this feature in other NI product lines.

Note: Because the internal pull-up resistor on these USB devices is small (4.7 kΩ) to pull the voltage below 0.8V, a pull-down resistor of 850 Ω or less is required. With such a small resistor  ~6 mA will be pulled from the device when driving the line high with no load connected. Since the device can only supply up to 8.5 mA, that only leaves ~2.5 mA for the load.  It is therefore not recommended to use this method to pull the voltage down to a low state. 

[Thomas_GERARDIN]

Hello Louis,

Using NI MAX : Ok I can create a task and execute it, then the port is in open-drain mode, but it is not setting "by default" the port. So when I disconnect / reconnect the DAQ, the port is once again in push-pull mode (5V on each pin).

Same for LabVIEW, it is not putting by default the port.

Best regards,

Thomas

[L.Zakarian]

Hello Thomas,

Unfortunately, this feature is not possible with the USB-6501 as described here (link given by ZYOng):

USB-6008/6009/6501

These USB devices have a default power up state of a high impedance input or alternatively called open collector/open-drain.
To set other values on the DIO lines, it must be statically updated by Software after power up of your system. If you have a need for programmable power-up states in your application, please look for this feature in other NI product lines.

---

If you really want to have programmable power-up state, you will then probably need to use one of the devices listed in this page.

I hope you will find a solution,

Best Regards

Louis Zakarian

[Thomas_GERARDIN]

Hello Louis,

Yes, it seems so. However, the price range for other devices is not the same.

Furthermore, installing a pull-down resistor doesn't seem like a good idea given the existing pull-up resistor (4.7K).

I could perhaps try replacing this pull-up resistor, but I suppose that I would lose the NI warranty.

Best regards,

Thomas

[L.Zakarian]

The price is indeed very different.

However, I really can't encourage you to replace the resistors yourself, as you would indeed void the NI warranty.

Best regards,

Louis Zakarian